1. Field of the Invention
The present invention relates generally to the field of cold forming tubular materials and, more particularly, to an apparatus and method for hydroforming a complex-shape frame from a blank tube.
2. Description of the Related Art
Industry requires standard blank tubes to be formed into one-piece, complex tubular shapes. In the automobile industry, automobile frames are typically of the "box" type construction for strength and load bearing purposes. These frame members often have a great variation in both the horizontal and vertical profile. The cross-section of such members often varies rather extremely from approximately a square cross-section, to a rectangular cross-section to a round cross-section to a severely flattened cross-section, and to any irregularly shaped combination of the above. The same is true for the antenna industry, which requires a wide variety of cross-section shapes for waveguides.
The general operations of bending, stretching, depressing and radially expanding a tube blank, with or without a mandrel, are known. For the majority of metals, it is fairly easy to bend small diameter tubing into an arc having a large radius. But as the diameter of the tubing increases and the radius about which it is to be bent decreases, the tube bending process requires some combination of compression at the inner bending radius of the tube and stretching at the outer radius. Although the outer bending surface of the tube may be stretched to the full extent of the materials rated elongation characteristics, a tube with a given diameter cannot be satisfactorily bent about a relatively small bending radius without encountering severe buckling at the inner bending surface or undesirable deformation at the outer bending radius. Some have achieved bending tubes with a certain diameter about relatively small bending radii by controllably dimpling or allowing controlled rippling of the inner tube surface thereby creating less stretching of the outer tube surface.
A standard mechanical press is one device used to shape blank tubes. FIG. 1a and 1b illustrate the standard mechanical press 10. The mechanical press 10 has a stationary lower die 12 supported by a fixed lower die bed 16. As shown in FIG. 1b, a blank tube 20 is placed into the cavity in the lower die 12. To shape the blank tube 20, an upper die 14 moves downward propelled by a ram press 18. The ram press 18 provides a force necessary to compress the blank tube 12 between the contacting lower and upper dies 12 and 14. The main problem with using a mechanical press to shape a blank tube 20 is that the depressed tube will not be pushed into the deep recesses of the cavity, especially for complex shapes. Since the depressed tube does not fill the recesses of the cavity, the shaped tube does not conform to the desired shape provided by the cavity between the lower and upper dies 14 and 20.
An apparatus that forms complex tubular shapes is a hydroforming press. The hydroforming press follows a series of steps to form the desired tubular shape. Generally, a tube or workpiece is placed between a pair of dies having cavities which defined the desired resultant shape of the tube. The dies merge, and the ends of the workpiece are sealed with a pair of sealing units. The workpiece is filled with fluid which is then pressurized. Pressurizing the fluid within the workpiece results in forming and expanding the tube to conform to the cavity shape. The fluid is drained from the tube and the sealing units are removed to release the workpiece. The main problem with the hydroforming press is its extreme cost. A single hydroforming press can cost approximately three million dollars.
Since mechanical presses are widely available and have been in service in many factories for years, attempts have been made to modify the mechanical presses to perform the above hydroforming operation. In transforming a standard mechanical press into a hydroforming press, sealing units must be added to seal the ends of the blank tube. The ram press lowers and stops the upper die at its lowered position. The sealing units supply the blank tube with a forming fluid which is then pressurized. Pressurizing the forming fluid within the blank tube forms and expands the blank tube to conform to the cavity shape. After the shaped tube is formed, the forming fluid is drained from the tube and the sealing units are removed to release the formed tube
The main problem with the mechanical press turned hydroformer is that when the upper die is lowered and stopped, the upper die does not contact the lower die to close the cavity between the dies. The ram press follows an elliptical path downward on its journey to have the upper die contact the lower die. Because the lower die is fixed, the ram press must stop its motion exactly when the two dies contact. However, the tolerance on a standard mechanical press leaves the ram press stopping at plus or minus five degrees from its one hundred and eighty degree point in which the dies would be in closed contact. Since the dies are unlikely to be completely closed when the tube is pressurized, the tube expanding under internal pressure to fill the deep recesses of the cavity also pinches between the mating dies. The end product from the transformed mechanical is an ill formed tube with the tube having ribs conforming to the space between the two non-contacting dies.
The present invention is directed to overcoming or at least reducing the effects of, one or more of the problems set forth above.